A friction material to be used for brakes such as disc brakes and drum brakes, clutches or the like is composed of raw materials such as a fiber base material for exerting a reinforcing function, a friction modifier for imparting a friction function and adjusting its friction performance and a binder for integrating these components.
With recent high-performance and speeding-up of vehicles, the role of brakes has become increasingly severe, and it is necessary to have a sufficiently high friction coefficient (effectiveness). Further, the temperature increases during braking at high speed, different from a frictional state during braking at low temperature and low speed, so that stable frictional properties having small changes in friction coefficient due to changes in temperature have been desired.
At present, it has been conventional that incorporation of an appropriate amount of metal fiber into a general friction material is effective for reinforcing the strength of the friction material, stabilizing the friction coefficient thereof, further maintaining the friction coefficient and improving a heat radiation effect at high temperature, improving abrasion resistance, and the like. Patent Document 1 discloses a friction material containing 5 to 10% by mass of a steel fiber, 5 to 10% by mass of a copper fiber having an average fiber length of 2 to 3 mm, and 2 to 5% by mass of a zinc powder having a particle diameter of 5 to 75 μm.
According to Patent Document 1, when the friction material contains the copper fiber in an amount within a given range, the friction coefficient at low temperature can be improved, and a decrease in friction coefficient at high temperature and high speed can be avoided. As the mechanism for this, it is assumed that during friction between the friction material and a counterpart material (disc rotor), an adhesion coating film is formed on a surface of the counterpart material due to the extensibility of the metal fiber contained in the friction material, and acts as a protective film, thereby largely contributing to stabilizing the friction coefficient at low temperature and maintaining the high friction coefficient at high temperature.
At present, therefore, the metal component contained in the friction material is mainly metal fiber such as a steel fiber or a copper fiber, in many cases. When these fibers are contained in large amounts, there is a possibility to cause abnormal wear of the above-mentioned disc rotor. That is, a wear debris of the disc rotor or a metal component contained in the friction material of the brake pad is incorporated into the friction material and aggregates therein to form large metal masses, which remain between the brake pad and the disc rotor, in some cases. The metal masses thus formed by aggregation sometimes abnormally wear the disc rotor (Patent Document 2).
Further, the copper component contained in the friction material is discharged as a wear debris by braking, so that the influence thereof on the natural environment has been pointed out.
Then, as a friction material containing no copper, Patent Document 3 discloses that even when the copper content is 0.5% by mass or less, all of high abrasion resistance during braking at high temperature, ensuring of a stable friction coefficient, and noise resistance are achieved by containing partially-graphitized coke and muscovite in given amounts in the friction material. In addition, Patent Document 4 discloses that even when the copper content is 0.5% by mass or less, it is possible to exhibit a stable friction coefficient and abrasion resistance and reduce squealing after leaving in a cold environment by allowing fluorine-based polymer particles to be contained in a friction material composition.
Patent Document 5 discloses that even when the content of copper and metal fiber other than copper is equal to or less than a predetermined value, a friction material having an excellent abrasion resistance at high temperature and little formation of metal catch is obtained by containing a titanate in a specific amount and further containing a zinc powder, in a friction material composition.